In recent years, highly conductive lithium salts such as lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate and lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(perfluoroethanesulfonyl)imide and lithium tris(trifluoromethanesulfonyl)methide have found frequent use in liquid, polymer and gel electrolytes for lithium primary and secondary batteries. See, for example, Kirk-Othmer's Encyclopedia of Chemical Technology, Fourth Edition, 3, 1016-1018 (1992) and 1107-1109; and 15, 446-447 (1995). Typically, liquid electrolytes for lithium batteries are made by dissolving lithium salt(s) of choice in anhydrous polar aprotic liquid solvent(s) at a Li.sup.+ molar concentration of around 0.5-2.0 M to produce a homogeneous solution having good conductivity and stability. The solvent must be sufficiently polar to effectively dissolve and dissociate the electrolyte salt, yet the solvent must be aprotic, i.e., free of any active hydrogen, to prevent reaction with the anode, which contains lithium metal or a form of carbon, such as graphite, intercalated with lithium. Liquid electrolytes are often very viscous, due to extensive polar interaction, and have a very high surface tension, exceeding 40 dynes/cm.
The liquid electrolyte is normally imbibed into the battery during the last part of its construction, and it is desirable that the electrodes and separator be quickly and thoroughly wet by the electrolyte so as to facilitate rapid battery manufacturing and to optimize battery performance. However, due to high viscosity and surface tension, the liquid electrolyte often cannot wet the separator or composite electrodes quickly and effectively. Separators are typically constructed from microporous polyolefin films which can have a surface energy as low as 30-35 dynes/cm. Electrodes are also frequently constructed from hard-to-wet (i.e., low surface energy) materials, including polytetrafluoroethylene and polyvinylidene fluoride binders. The very small size and tight construction of most lithium batteries (typically button, jelly-roll or prismatic configurations) further aggravates the wetting problem.
The relative surface energy between a liquid and a porous solid is very important in determining the wetting properties of the liquid. A liquid with a surface energy higher than the surface energy of the solid substrate will not wet the solid. The liquid will bead on the surface of the solid. A liquid with a surface energy approximately equal to the surface energy of the solid substrate will wet the solid but at a slow rate of penetration and will only penetrate the larger pores of the solid. A liquid with a surface energy lower than that of the solid substrate will rapidly wet the solid and penetrate substantially all of the open porosity of the solid. Therefore in order to effect complete and rapid wetting of the separator and electrode materials of a battery, the surface energy of the liquid should be less than the surface energy of the solid materials. These wetting properties apply not only to pure liquids but also to materials with liquid phases such as plasticized polymers.
Many tradeoffs are made in battery design and process engineering in order to accommodate the necessity of a complete and rapid electrolyte fill operation. For example, the electrodes cannot be manufactured to minimize porosity because at some point the pore size will be too small to be effectively wetted by electrolyte. Yet the lower the porosity of the electrode the more active material that can be packed into the cell and the higher the resulting energy of the battery. As a result, increasing the wetting properties of an electrolyte will allow the use of electrodes with higher density and energy.
Many of the electrolyte formulations available have a surface energy, which is too high to spontaneously wet the battery components. These formulations must be compromised with suitable solvents, which decrease the performance characteristics of the battery. The use of a surfactant will allow the use of electrolyte solvent formulations not previously accessible to the battery engineer.
Special process techniques are sometimes employed such as vacuum or pressure to accelerate the wetting of components by the electrolyte. Increasing the wetting properties of the electrolyte can minimize or eliminate these techniques and can decrease the time necessary for the electrolyte fill operation.
The availability of a compatible surfactant salt opens up a wider range of operating parameters for the battery engineer in the design and manufacture of components, selection of materials, and formulation of electrolytes.
Conventional surfactants which aid electrolyte wetting can have an adverse effect on cell performance, due to their inherent thermal or redox instability, their interference with conductivity, or their incompatibility with other cell components such as the anode, cathode or current collector.
Thus, there remains a need to improve the wetting of battery electrodes and separators by a non-aqueous liquid electrolyte while maintaining the desired stability and compatibility of the liquid electrolyte with other cell components.